1.Several species of Annelida were examined on the infection of acephaline gregarines. Three of them. Eisenia foetida SAVIGNY,Pontodrilus matsushimensis IZUKA,Pheratima communissima GOTO et HATAI were infected with 3 Apolocystis, 2 Nematocystis and 1 Monocystidae gregarines. With the characters, all of them were undescribed species in Japan untill now.2.Seasonal changes of infection rate and body length of acephaline gregarines parasitized in Eisenia foetida. The infection rate become high from spring to early summer but decrease from midsummer. The seasonal chage of the body length is small but the average of the body length is rather large in witer.3.The ultrastructure of the body surface, nucleus and mucron were examined by TEM.and SEM.The ultrastructure of periphery and hair-like projection of Filipodium ozakii was studied. The appearance of projection resembled cilia or flagella but their ultrastructure was characteristic and completely different from them. The nucleus of Odonaticola was isolated from the body and observed by TEM and SEM.The surface was covered with the fine fibrous net.4.The cross infection of Gregarina blattarum was studied amoung several different species of the hosts. The cross infection was not occured among Blatella germanica Periplaneta fuliginosa, Perilaneta japonica and Periplaneta americana. The cross infection was only occured between Blatella germanica and Blatella nipponica.